The present invention is generally concerned with a printing machine construction, and more particularly is directed to an improvement in the transfer of sheets between a first and a second printing roller of a printing machine, which respectively print two opposite sides of a sheet.
The concept of transferring a sheet from one printing roller to a succeeding one, and reversing the sheet side-for-side so that each of the printing rollers will print on a different side of the sheet, is already known, for example, from DDR Pat. No. 54,703. In this prior-art patent the sheet is engaged at the tangent point defined by the transfer roller and the first printing roller, by means of a suction gripper system which upon engagement moves inwardly of the outline of the transfer roller and yields the sheet to a mechanical gripper system. The mechanical gripper system, which at this time also is in a position in which it is pivoted inwardly of the outline of the transfer roller, now pivots outwardly of the outline and, during continued rotation of the transfer roller, offers what was previously the trailing edge of the sheet (while the same was still on the first printing roller) as the leading edge to the gripper system of the second printing roller, so that the sheet becomes reversed side-for-side and the second printing roller is now able to print upon the side of the sheet opposite the one that was printed by the first printing roller.
The suction system is composed of a plurality of individual suction grippers which are distributed over the axial length of the transfer roller and intermediate which there are free spaces into which the mechanical grippers engage. The holding force exerted by the suction gripper system upon the trailing edge of the sheet as it picks the same off the first printing roller is limited by the degree of vacuum the surface area over which the suction engagement takes place and the coefficient of friction between the individual suction grippers and the paper of the sheet. Particularly at high operating speeds this suction effect may not be sufficient to withstand the forces that act upon it, such as the inertia of the sheet during the phase during which the latter is accelerated by the transfer roller, the force with which the sheet tends to adhere to the surface of the first printing roller, the frictional force that must be overcome between the sheet and the first printing roller as the sheet is pulled off the surface of the printing roller, the force of gravity and the air resistance. Because of this, when certain types of paper are used at high operating speeds, for example 8,000 sheets per hour, the differences in the sheet position with reference to the sheet guiding elements may result. The holding force of the suction gripper system is particularly limited due to the small surface area of engagement between the sheet and the individual suction grippers, since only approximately 50% of the trailing edge of the sheet will be in engagement with the suction grippers.